A circle has a radius of $3$. An arc in this circle has a central angle of $\dfrac{7}{4}\pi$ radians. What is the length of the arc? ${6\pi}$ ${\dfrac{7}{4}\pi}$ $\color{#DF0030}{\dfrac{21}{4}\pi}$ ${3}$
First, calculate the circumference of the circle. $c = 2\pi r = 2\pi (3) = 6\pi$ The ratio between the arc's central angle $\theta$ and $2 \pi$ radians is equal to the the ratio between the arc length $s$ and the circle's circumference $c$ $\dfrac{\theta}{2 \pi} = \dfrac{s}{c}$ $\dfrac{7}{4}\pi \div 2 \pi = \dfrac{s}{6\pi}$ $\dfrac{7}{8} = \dfrac{s}{6\pi}$ $\dfrac{7}{8} \times 6\pi = s$ $\dfrac{21}{4}\pi = s$